Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” was originally performed using wireline logging.
In wireline logging, an operator lowers a probe or “sonde” into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, the sonde measures various parameters of the Earth's formations and correlates them with the sonde's position as the operator pulls it uphole.
Although useful, wireline logging does have its limitations. If the borehole has been cased, i.e., lined with steel casing that has been cemented in place, then the sensing abilities of most wireline tools may be impaired. An operator will often remove any tubulars in the borehole before performing a wireline logging run, thereby adding cost and delay to the logging process. Moreover, the delay often degrades the logging measurement quality due to migration of fluid from the borehole into the formation or due to caving and collapse of the borehole wall. Wall caving can potentially also trap the logging tool downhole.
Consequently, engineers have created other logging methods such as logging while drilling (“LWD”) or measurement while drilling (“MWD”). Such methods generally are unable to feasibly employ a logging cable because (if unprotected) the cable quickly gets pinched between the drillpipe and the borehole wall, resulting in the shearing or shorting out of the cable. Engineers have thus created various alternative wireless telemetry methods to communicate information between downhole tools and the surface. Such methods include, among others, electromagnetic (“EM”) telemetry.
As drilling progresses, however, the distance between a downhole logging tool and a surface system receiving the tool's EM telemetry steadily increases. The increased distance produces a corresponding increase in the attenuation of the communication signal between the logging tool and the surface system. This is because electromagnetic signals, even at very low frequencies, become attenuated as they propagate along the borehole. Such attenuation results in a reduced signal-to-noise ratio (SNR) of the received signal, making error-free detection and demodulation at the surface progressively more difficult. Increasing the power output of the logging tool's transmitter is generally not an option, as the maximum transmission power is limited by the logging tool's overall power budget. Further, while increased amplification of the received signal can help reduce the error rate under some circumstances, such increased amplification also further amplifies the noise, which can cause increased interference with the received signal as the SNR decreases.
It should be understood that the drawings and corresponding detailed description do not limit the disclosure, but on the contrary, they provide the foundation for understanding all modifications, equivalents, and alternatives falling within the scope of the appended claims.